Use of spike tires has been banned by law in Japan so as to prevent powder dust pollution resulting from the spike tires, and thus studless tires are now used instead of the spike tires in cold regions. In order to improve grip performance of the studless tires on ice or snow, there is a method that decreases an elastic modulus at low temperatures and improves traction. Particularly, the braking force on ice is largely affected by an effective-contact area between rubber and ice. In order to enlarge the effective-contact area, a rubber has been desired which is flexible at low temperatures.
On the other hand, where the hardness of rubber alone is decreased using a method that comprises, for example, increasing the amount of oil, studless tires for passenger vehicles have a problem that the handling stability decreases.
Generally, the tread rubber of a studless tire, not only for trucks, buses and light trucks but also for passenger vehicles, contains a natural rubber or a butadiene rubber as a main ingredient and is produced through vulcanization by adding sulfur as a vulcanizing agent in an amount of about 1.5 parts by mass per 100 parts by mass of a rubber component in many cases (for example, see JP 2007-176417 A). This is because these rubbers have a low glass transition temperature and flexibility even though they have high strength. However, reversion occurs when the natural rubber or butadiene rubber is sulfur-vulcanized. In this phenomenon, rubber degrades or its cross-linked state deteriorates, so that the elastic modulus at low temperatures also decreases. However, the inventors of the present invention have found from their studies that the hardness also excessively decreases and thus the handling stability decreases.
Recently, in many cases, tires are produced through vulcanization at high temperatures not less than 170° C. in order to improve the productivity of tires. However, it has been found that the reversion often occurs when the vulcanization is performed at such high temperatures; and this causes decrease in the handling stability. In addition, the reversion may decrease the abrasion resistance and increase tan δ at high temperatures excessively, thereby decreasing fuel economy which is a particularly important property.